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Add system view for shared buffer lookup table
The view exposes the contents of the shared buffer lookup table for debugging, testing and investigation. TODO: It is better to place this view in pg_buffercache. But it's added as a system view since BufHashTable is not exposed outside buf_table.c. To move it to pg_buffercache, we should move the function pg_get_buffer_lookup_table() to pg_buffercache which invokes BufTableGetContent() by passing it the tuple store and tuple descriptor. BufTableGetContent fills the tuple store. The partitions are locked by pg_get_buffer_lookup_table(). Author: Ashutosh Bapat <[email protected]>
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‎doc/src/sgml/system-views.sgml‎

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@@ -71,6 +71,11 @@
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<entry>backend memory contexts</entry>
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</row>
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<row>
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<entry><link linkend="view-pg-buffer-lookup-table"><structname>pg_buffer_lookup_table</structname></link></entry>
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<entry>shared buffer lookup table</entry>
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</row>
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<row>
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<entry><link linkend="view-pg-config"><structname>pg_config</structname></link></entry>
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<entry>compile-time configuration parameters</entry>
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</para>
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</sect1>
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<sect1 id="view-pg-buffer-lookup-table">
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<title><structname>pg_buffer_lookup_table</structname></title>
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<indexterm>
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<primary>pg_buffer_lookup_table</primary>
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</indexterm>
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<para>
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The <structname>pg_buffer_lookup_table</structname> view exposes the current
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contents of the shared buffer lookup table. Each row represents an entry in
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the lookup table mapping a relation page to the ID of buffer in which it is
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cached. The shared buffer lookup table is locked for a short duration while
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reading so as to ensure consistency. This may affect performance if this view
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is queried very frequently.
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</para>
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<table id="pg-buffer-lookup-table-view" xreflabel="pg_buffer_lookup_table">
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<title><structname>pg_buffer_lookup_table</structname> View</title>
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<tgroup cols="1">
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<thead>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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Column Type
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</para>
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<para>
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Description
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</para></entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>tablespace</structfield> <type>oid</type>
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</para>
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<para>
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OID of the tablespace containing the relation
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>database</structfield> <type>oid</type>
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</para>
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<para>
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OID of the database containing the relation (zero for shared relations)
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>relfilenode</structfield> <type>oid</type>
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</para>
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<para>
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relfilenode identifying the relation
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>forknum</structfield> <type>int2</type>
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</para>
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<para>
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Fork number within the relation (see <xref linkend="storage-file-layout"/>)
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>blocknum</structfield> <type>int8</type>
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</para>
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<para>
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Block number within the relation
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>bufferid</structfield> <type>int4</type>
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</para>
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<para>
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ID of the buffer caching the page
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</para></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Access to this view is restricted to members of the
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<literal>pg_read_all_stats</literal> role by default.
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</para>
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</sect1>
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<sect1 id="view-pg-config">
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<title><structname>pg_config</structname></title>
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‎src/backend/catalog/system_views.sql‎

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@@ -1420,3 +1420,10 @@ REVOKE ALL ON pg_aios FROM PUBLIC;
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GRANT SELECT ON pg_aios TO pg_read_all_stats;
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REVOKE EXECUTE ON FUNCTION pg_get_aios() FROM PUBLIC;
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GRANT EXECUTE ON FUNCTION pg_get_aios() TO pg_read_all_stats;
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CREATE VIEW pg_buffer_lookup_table AS
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SELECT * FROM pg_get_buffer_lookup_table();
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REVOKE ALL ON pg_buffer_lookup_table FROM PUBLIC;
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GRANT SELECT ON pg_buffer_lookup_table TO pg_read_all_stats;
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REVOKE EXECUTE ON FUNCTION pg_get_buffer_lookup_table() FROM PUBLIC;
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GRANT EXECUTE ON FUNCTION pg_get_buffer_lookup_table() TO pg_read_all_stats;

‎src/backend/storage/buffer/buf_table.c‎

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*/
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#include "postgres.h"
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#include "fmgr.h"
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#include "funcapi.h"
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#include "storage/buf_internals.h"
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#include "storage/lwlock.h"
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#include "utils/rel.h"
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#include "utils/builtins.h"
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/* entry for buffer lookup hashtable */
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typedef struct
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if (!result) /* shouldn't happen */
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elog(ERROR, "shared buffer hash table corrupted");
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}
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/*
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* SQL callable function to report contents of the shared buffer lookup table.
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*/
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Datum
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pg_get_buffer_lookup_table(PG_FUNCTION_ARGS)
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{
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#define PG_GET_BUFFER_LOOKUP_TABLE_COLS 6
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ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
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HASH_SEQ_STATUS hstat;
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BufferLookupEnt *ent;
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Datum values[PG_GET_BUFFER_LOOKUP_TABLE_COLS];
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bool nulls[PG_GET_BUFFER_LOOKUP_TABLE_COLS];
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int i;
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memset(nulls, 0, sizeof(nulls));
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/*
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* We put all the tuples into a tuplestore in one scan of the hashtable.
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* This avoids any issue of the hashtable possibly changing between calls.
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*/
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InitMaterializedSRF(fcinfo, 0);
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Assert(rsinfo->setDesc->natts == PG_GET_BUFFER_LOOKUP_TABLE_COLS);
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/*
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* Lock all buffer mapping partitions to ensure a consistent view of the
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* hash table during the scan. Must grab LWLocks in partition-number order
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* to avoid LWLock deadlock.
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*/
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for (i = 0; i < NUM_BUFFER_PARTITIONS; i++)
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LWLockAcquire(BufMappingPartitionLockByIndex(i), LW_SHARED);
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hash_seq_init(&hstat, SharedBufHash);
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while ((ent = (BufferLookupEnt *) hash_seq_search(&hstat)) != NULL)
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{
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values[0] = ObjectIdGetDatum(ent->key.spcOid);
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values[1] = ObjectIdGetDatum(ent->key.dbOid);
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values[2] = ObjectIdGetDatum(ent->key.relNumber);
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values[3] = ObjectIdGetDatum(ent->key.forkNum);
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values[4] = UInt32GetDatum(ent->key.blockNum);
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values[5] = Int32GetDatum(ent->id);
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tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
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values, nulls);
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}
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/*
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* Release all buffer mapping partition locks in the reverse order so as
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* to avoid LWLock deadlock.
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*/
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for (i = NUM_BUFFER_PARTITIONS - 1; i >= 0; i--)
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LWLockRelease(BufMappingPartitionLockByIndex(i));
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return (Datum) 0;
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}

‎src/include/catalog/pg_proc.dat‎

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proargmodes => '{o,o,o}', proargnames => '{name,type,size}',
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prosrc => 'pg_get_dsm_registry_allocations' },
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# buffer lookup table
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{ oid => '5102',
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descr => 'shared buffer lookup table',
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proname => 'pg_get_buffer_lookup_table', prorows => '6', proretset => 't',
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provolatile => 'v', prorettype => 'record',
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proargtypes => '', proallargtypes => '{oid,oid,oid,int2,int8,int4}',
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proargmodes => '{o,o,o,o,o,o}',
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proargnames => '{tablespace,database,relfilenode,forknum,blocknum,bufferid}',
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prosrc => 'pg_get_buffer_lookup_table'
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},
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# memory context of local backend
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{ oid => '2282',
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descr => 'information about all memory contexts of local backend',

‎src/test/regress/expected/rules.out‎

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@@ -1330,6 +1330,13 @@ pg_backend_memory_contexts| SELECT name,
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free_chunks,
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used_bytes
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FROM pg_get_backend_memory_contexts() pg_get_backend_memory_contexts(name, ident, type, level, path, total_bytes, total_nblocks, free_bytes, free_chunks, used_bytes);
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pg_buffer_lookup_table| SELECT tablespace,
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database,
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relfilenode,
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forknum,
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blocknum,
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bufferid
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FROM pg_get_buffer_lookup_table() pg_get_buffer_lookup_table(tablespace, database, relfilenode, forknum, blocknum, bufferid);
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pg_config| SELECT name,
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setting
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FROM pg_config() pg_config(name, setting);

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